This invention relates to computer systems and more particularly to such systems that employ one or more internal air fans for cooling the internal electrical heat generating components of the systems. In computer systems that are required to operate for long periods of time or continuously, for example servers, which systems usually employ an array of fans, a great concern is the effect a failed fan may have in allowing over or prohibitive heating of the components resulting in a reduction in performance and even their failure. In a system employing an array of adjacent fans, for example, where the array is employed in a rack containing a large number of electrical components, the failure of a single fan can result in an objectionable over heating of the components. While the computer system of the failed fan may be provided with well known temperature and fan rotation sensors, alarms and heat shut off devices, some of these protective devices require immediate attention or effect a shutting down of the system, which may not be practical or an acceptable option. For these reasons it is a common practice to provide a redundancy fan system, which while solving one set of problems is not without some serious drawbacks, such as additional cost, the use of internal computer space where space in modern computers is at an ever increasing premium, complexity in control and performance of the fans and unpredictable flow rate thereof.
The ever increasing power densities of modern electronics being installed in today's computer systems create an ever higher impedance to airflow. This results in the creation of a high static pressure condition at the outlet side of the fan or fans. As a result of this condition, air in the form of recirculating air at the outlet side of a failed fan is forced in a direction reversed from the normal airflow of the fan due to the lower impedance the opening of the failed fan offers. The recirculating air at the inlet side of the failed fan is thus allowed to mingle with the inlet air of an adjacent operating fan causing a reduction in the air mass and cooling capacity available to be circulated over the electrical components by the other fan. In addition, the recirculated air when reintroduced by the operating fan to effect cooling of the components will continue to gain heat adding to the problem.